Synthetic nucleic acids with antisense sequence complementary to mRNA, and their use for gene activity detection, have advanced our understanding of the molecular mechanisms of diseases in all disciplines of the biological sciences. For in vivo investigations, oligoDNA (ODN) or oligoRNA (ORN) can be modified with phosphorothioate (yielding sODN or sORN) to increase resistance to nucleases. Our hypothesis is that hybrids of sORN with target mRNA is more stable than sODN hybrids. We will compare a modular magnetic resonance (MR) probe comprising supraparamagnetic iron oxide nanoparticles (SPION, a T2 agent) labeled with sODN or sORN. At present, intracerebroventricular (ICV) injection via cortical or lumbar puncture is one of only a few clinically approved methods to deliver drugs to the cerebral spinal fluid (CSF) in humans. We have demonstrated that neural cells of live animals take up SPION-sODN with moderate efficiency and specificity in mRNA targeting in vivo by MR imaging: (1) ICV delivery in mice safely facilitates global distribution of SPION- sODN in mouse brains without lethal effect, (2) specific binding has been shown by in vivo priming of SPION- sODN to target mRNA by reverse transcription (RT), (3) results from electron microscopy (EM) show that iron oxide is located in the end some with a unique association to the endoplasmic reticulum (ER) and nuclei where mRNA is located, (4) changes in SPION-sODN retention above baseline (DR2*) are positively proportional to gene activities (linear regression = 1.0). Our goal is to evaluate the efficiency of SPION-sORN (and SPION-sODN) for targeting astroglia-specific glial fibrillary acidic protein (GFAP) mRNA. Completion of the proposed work provides a platform for novel gene targeting probes as well as a powerful tool for early evaluation of astroglia activation in vivo. Therefore, less SPION-sORN than SPION-sODN is used for gene targeting and reduces accumulation of iron in the brain, leading to longitudinal assessment of neurologic events. We will: Aim 1: Compare in vivo dose and uptake of SPION-sODN or SPION-sORN in mice using ultra-high field MRI. Our hypothesis is that SPION retention (DR2*) will improve when SPION-sORN (SPION-Rgfap) is used to target GFAP mRNA. We will longitudinally compare DR2* of these two probes in the brains of live mice. Aim 2: Validate the correlation between MRI and histological assessments. Our hypothesis is that co- localization of dual-labeled probe (e.g., SPION-Rgfap-Cy3) can be specifically transfected to GFP-expressing glia of transgenic mice in vivo, and can be confirmed under fluorescent, optical and electron microscopes. We will collect brain samples after ICV probe delivery for this correlation study. Aim 3: Validate target binding using primer-free in situ RT to cDNA followed by target specific PCR. The hypothesis is that SPION-Rgfap will bind specifically to GFAP mRNA target in vivo and serve as a primer for in situ RT-PCR. We will collect brain samples, quantify the PCR results, and establish the correlation between MRI DR2* and mRNA copy numbers, using disease model systems.